1. Field of the Invention
The present invention relates generally to the field of equalizers for use in optical communications networks. More specifically, the present invention discloses a dynamic gain equalizer for use primarily with erbium-doped fiber amplifiers.
2. Statement of the Problem
Erbium-doped fiber amplifiers (EDFA) are widely used in optical communication systems. However, their gain spectrum is not flat, which limits their applications in dense wavelength division multiplex (DWDM) systems. To address this problem, some EDFAs use a fixed filter in an attempt to flatten the gain spectrum. For example, a typical commercial EDFA has a gain ripple of approximately 1-3 dB using fixed filters. As shown in FIG. 1, a typical EDFA gain spectrum will shift as a function of its gain setting. While a fixed filter may be effective at a specific operating gain for an EDFA, a fixed filter cannot accommodate changes in the gain spectrum as illustrated in FIG. 1. Future WDM systems will require EDFAs with operating windows of more than 30 nm and gain flatness within 1 dB peak-peak. It is also essential for these high performances to be maintained over a range of operating gains and over the lifetime of the EDFA.
The prior art in this field includes several approaches that have been introduced to implement dynamically adjustable filters. Acousto-optic tunable filters have been used to flatten an EDFA to xc2x10.7 dB over a 6 dB dynamic range as taught by Hyo Sang Kim et al., xe2x80x9cDynamic Gain Equalization of Erbium-Doped Fiber Amplifier With All-Fiber Acousto-Optic Tunable Filters,xe2x80x9d OFC ""98 Technical Digest, WG4, pp. 136-138. However, acousto-optic tunable filters have the drawbacks of significant polarization sensitivity, intermodulation effects produced by the multiple drive frequencies, and high RF power consumption.
Parry et al., xe2x80x9cDynamic Gain Equalisation of EDFAs with Fourier Filters,xe2x80x9d Tech. Dig. OAA, 1999, paper ThD 22 (Nortel) have suggested that a set of harmonic sinusoidal filter elements, (e.g., Mach-Zehnder devices) can be cascaded together to build a dynamic gain flattener if each filter can tune center frequencies and attenuation depths, as taught by Betts et al. xe2x80x9cSplit-Beam Fourier Filter and its Application in a Gain-Flattened EDFA,xe2x80x9d OFC ""95 Technical Digest, TuP4, pp. 80-81.
3. Solution to the Problem
The present invention employs liquid crystal light modulator technology to implement a dynamic gain equalizer consisting of a series of sinusoidal filters. Nothing in the prior teaches or suggests an equalizer using liquid crystal technology to implement a sequence of sinusoidal filters with tunable depth and center wavelength. In contrast to the prior art, the present approach offers the following advantages:
1. Dynamic. The filters are able to flatten the gain profile at different input levels, different temperatures, and different periods in the amplifier""s life time.
2. High performance. The present device has low insertion loss, low polarization dependent loss, fast response and low power consumption.
3. Easy to implement. The present design consists of a series of LC cells and crystals. Liquid crystal cells are a mature technology and the requirements for the LC cells are within industrial standards. Crystal polishing and cutting is also a mature technology.
4. Cost effective. All parts in the present design are neither expensive nor difficult to obtain.
This invention provides an optical equalizer having an initial polarizer that convert the input beam to a predetermined polarization, followed by a series of dynamically-adjustable sinusoidal filters that provide attenuation as a sinusoidal function of beam wavelength. Each of the sinusoidal filters has a first liquid crystal cell adjustably rotating the polarization of the beam from the preceding polarizer. This is followed by a second optical element that retards the beam as a sinusoidal function of beam wavelength. For example, the second optical element can be a birefringent crystal that provided a fixed degree of retardance to the beam and a second liquid crystal cell that provides a variable degree of retardance, thereby allowing adjustment of the center frequency of the sinusoidal function. Finally, a third liquid crystal cell adjustably rotates the polarization of the beam. A final polarizer provides amplitude control of the beam based on the polarization rotations introduced by the first and third liquid crystal cells. A controller provides control signals to the liquid crystal cells of each sinusoidal filter so that their combined sinusoidal attenuation functions produce a desired equalization curve.
These and other advantages, features, and objects of the present invention will be more readily understood in view of the following detailed description and the drawings.